Stabilized lithium metal powder for Li-ion application, composition and process

ABSTRACT

The present invention provides a method for stabilizing lithium metal powder. The method comprises the steps of heating the lithium metal powder to above its melting point to provide molten lithium metal, dispersing the molten lithium metal, and contacting the dispersed molten lithium metal with a phosphorous-containing compound to provide a substantially continuous protective layer of lithium phosphate on the lithium metal powder.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/938,284, filed May 16, 2007, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to stabilized lithium metal powder(“SLMP”) having better stability and a longer storage life. Suchimproved SLMP can be used in a wide variety of applications includingorgano-metal and polymer synthesis, rechargeable lithium batteries, andrechargeable lithium ion batteries.

BACKGROUND OF THE INVENTION

The highly reactive, i.e., pyrophoric, nature of lithium metal,particularly high surface area lithium metal powder, can be a deterrentfor its use in a variety of applications. Thus lithium metal istypically in a stabilized form. It is known to stabilize lithium metalpowder by passivating the metal powder surface with CO₂ such asdescribed in U.S. Pat. Nos. 5,567,474, 5,776,369, and 5,976,403, thedisclosures of which are incorporated herein in their entireties byreference. The CO₂ passivated lithium metal powder, however, can be usedonly in air with low moisture levels for a limited period of time beforethe lithium metal content decays because of the reaction of the lithiummetal and air.

Another option has been to coat the lithium powder with a protectivelayer. For example, U.S. Pat. No. 6,911,280B1 proposes coating with analkali or alkaline earth metal carbonate. U.S. Pat. No. 4,503,088proposes coating an epoxy resin on a lithium negative electrode as apassivation layer. U.S. Pat. Nos. 5,342,710 and 5,487,959 propose usinga complex of I₂ and poly-2-vinylpyridine as a passivation layer. Thesesuggested protective layers; however, often result in a decrease inconductivity and weak mechanical strength.

SUMMARY OF THE INVENTION

The present invention provides a method for stabilizing lithium metalpowder. The method comprises the steps of heating the lithium metalpowder to above its melting point to provide molten lithium metal,dispersing the molten lithium metal, and contacting the dispersed moltenlithium metal with a phosphorous-containing compound such as phosphoricacid to provide a substantially continuous protective layer of lithiumphosphate on the lithium metal powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparison of SEM images for stabilized lithium powderprepared according to Examples 1-4.

FIG. 2 is a comparison of SEM images for stabilized lithium powderprepared according to Examples 1 and 5.

FIG. 3 is an air stability comparison of Comparative Example 1,Comparative Example 2, and Example 1.

FIG. 4 is an Advanced Reactive Screening Tool Colorimeter (ARSST) Testcomparing the stability of Comparative Example 1 and Example 4.

FIG. 5 is a Vent Sizing Package 2 (VSP2) test comparing the stability ofExample 1 and Example 5 in NMP.

DETAILED DESCRIPTION OF THE INVENTION

In the drawings and the following detailed description, variousembodiments are described in detail to enable practice of the invention.Although the invention is described with reference to these specificembodiments, it will be understood that the invention is not limited tothese embodiments. But to the contrary, the invention includes numerousalternatives, modifications and equivalents as will become apparent fromconsideration of the following detailed description and accompanyingdrawing.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items. As used herein, the singularforms “a”, “an,” and “the” are intended to include the plural forms aswell, unless the context clearly indicates otherwise. It will be furtherunderstood that the terms “comprises” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. Furthermore,the term “about,” as used herein when referring to a measurable valuesuch as an amount of a compound or agent of this invention, dose, time,temperature, and the like, is meant to encompass variations of 20%, 10%,5%, 1%, 0.5%, or even 0.1% of the specified amount.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety.However, the citation of a reference herein should not be construed asan acknowledgement that such reference is prior art to the presentinvention described herein.

The present invention relates to a method of providing stable lithiummetal powder. The method includes the step of heating lithium metalpowder to above its melting point in an inert atmosphere. Typically thisis above about 200° C. Often this is done by heating a non-combustiblehydrocarbon oil. Exemplary hydrocarbon oils include mineral oil, or anyother saturated hydrocarbon solvent having a branched, straight chain orsaturated cyclic structures and a flash point above about 200° F. Avariety of hydrocarbon oils may be used in the present invention. Theterm hydrocarbon oil, as used herein, includes various oily liquidsconsisting chiefly or wholly of mixtures of hydrocarbons and includesmineral oils, i.e., liquid products of mineral origin having viscositylimits recognized for oils and hence includes but is not limited topetroleum, shale oils, paraffin oils and the like. There are manymanufacturers of these useful hydrocarbon oils. Among these usefulhydrocarbon oils are highly refined oils, such as, Peneteck manufacturedby Penreco Division of Pennzoil Products Inc., which has a viscosity inthe range of 43-59 pascal-sec at 100° F. and a flash point of 265° F.,Parol 100, which has a viscosity of 213-236 pascal-sec at 100° F. and aflash point of 360° F. (available from Penreco, Div. of PennzoilProducts), and Carnation white oil (viscosity=133-165 pascal-sec at 100°F.) made by Sonneborn Div. of Witco. Even certain purified hydrocarbonsolvents which boil in a range encompassing the melting point of lithiummay be used, such as UNOCAL's 140 Solvent. In addition, unrefined oils,such as Unocal's 460 Solvent and Hydrocarbon Seal oil and Exxon's Telura401 and Telura 407 may also be used. The selection of a hydrocarbon oilwill be within the skill of one in the art.

The molten lithium metal is then dispersed such as by agitating orstirring vigorously to apply high sheer forces. The dispersion stepusing high sheer or other equivalent forces is conducted to form uniformdroplets or particles of the lithium metal, and to facilitatedistributing the droplets or particles in the hydrocarbon oil whileavoiding agglomeration.

The dispersed molten lithium is contacted with a phosphorous-containingcompound such as phosphoric acid (H₃PO₄) to provide a substantiallycontinuous protective layer of lithium phosphate (Li₃PO₄) on the lithiummetal powder. Other phosphorous-containing compounds can be used,particularly if phosphoric acid is formed during its reaction with thelithium. For example, P₂O₅ can be used if reacted in the presence ofmoisture so that H₃PO₄ is formed first and then the Li₃PO₄ is formed.Alternatively POF₃ gas could be hydrolyzed into HF and H₃PO₄ which thenreacts with lithium to form Li₃PO₄.

The phosphorous-containing compound can be introduced to contact thelithium droplets during the dispersion at a temperature above thelithium melting point, or at a lower temperature after the lithiumdispersion has cooled. The phosphorous-containing compound could beintroduced in a crystalline form or in a solution form in mineral oil orany other suitable solvent. It is understood that combinations ofdifferent process parameters could be used to achieve specific coatingcharacteristics. For example, control of reaction rate between lithiumand the phosphorous-containing compound is essential in preventingcavities and or cracks being formed. Furthermore, it is beneficial tocombine the coating with an organic coating, for example, differenttypes of waxes with different chemical compositions, molecular weights,melting points and hardness could be used to achieve specific coatingcharacteristics for particular applications and the like, to improveboth air stability and polar solvent stability that would allow bothsafer handling and possibility of using commonly used polar solventsthat dissolve commonly used polymer binders.

Suitable waxes can be natural wax such as 12-hydroxystearic acid,synthetic wax such as low molecular weight polyethylene, petroleum waxessuch as paraffin wax, and microcrystalline waxes. The wax can beintroduced to contact the lithium droplets during the dispersion, or ata lower temperature after the lithium dispersion has cooled. It isunderstood that combinations of different types of waxes with differentchemical compositions, molecular weights, melting points and hardnesscould be used to achieve specific coating characteristics for particularapplications. For example, degree of stickiness could be controlled toallow introduction of the SLMP using a “transfer release paper” concept,wherein a certain degree of stickiness is required.

Suitable waxes described above could produce two types of coatings onlithium particles: first type representing physical or adhesive typewhere non-polar waxes are used and a second type, representingchemically bonded coatings where waxes with functional groups, havingboth hydrophobic and hydrophilic features, are used. The coatingthickness could vary in the range of about 20 nm to about 200 nm.

The present invention also provides a lithium metal powder protected byLi₃PO₄. A thin, dense, substantively continuous Li₃PO₄ layer of theinvention provides improved protection such as compared to typical CO₂and LiF passivation techniques. The lithium phosphate layer may compriseabout 0.5% to 20% by weight of the stabilized lithium metal powder. Thisrange is estimated based on a 45 micron particle: 0.01 micron coatingcorresponds to 0.74% Li₃PO₄ and 0.3 micron coating corresponds to 18.6%Li₃PO₄. The resulting lithium metal powder has improved stability andimproved storage life. To this end, a more stable lithium metal powderis provided. The lithium metal powder is passivated by H₃PO₄. Such athin, dense, continuous Li₃PO₄ layer provides better passivation ascompared to CO₂ and LiF because of the insolubility of Li₃PO₄ in water(i.e., 0.04 g in 100 g H₂O at 25° C.) vs. LiF (i.e., 0.133 g in 100 gH₂O at 25° C.) and Li₂CO₃ (i.e., 1.29 g in 100 g H₂O at 25° C.). TheLi₃PO₄ passivation layer serves as a better barrier against moisture andatmosphere gases.

The following examples are merely illustrative of the invention, and arenot limiting thereon.

EXAMPLES Example 1

Battery grade lithium metal (411 grams) was cut into 2×2 inch pieces andcharged under constant flow of dry argon at room temperature to a 3liter stainless steel flask reactor with a 4″ top fitted with a stirringshaft connected to a fixed high speed stirrer motor. The reactor wasequipped with top and bottom heating mantles. The reactor was assembledand 1078 g of Peneteck™ oil were added. The reactor was then heated toabout 200° C. and gentle stirring was maintained in the range of 250 rpmto 800 rpm to ensure all metal was molten. Then the mixture was stirredat high speed (up to 10,000 rpm) for 2 minutes. Oleic acid, 8.22 g wascharged into the reactor and high speed stirring continued for another 3minutes. Then the high speed stirring was stopped, the heating mantleswere removed and the dispersion was allowed to cool to about 46° C.Next, 21.4 grams of phosphoric acid melted in advance in 68.59 grams ofoil was charged into the reactor while stirring at about 800 rpm, thetemperature rise of 2° C. was noted. The dispersion was stirred foranother 10 minutes and then transferred to the storage bottles. Further,lithium dispersion was filtered and washed three times with hexane in anenclosed, sintered glass filter funnel and once with n-pentane to removethe hydrocarbon oil medium. The funnel was heated with a heat gun toremove traces of the solvents and the resulting free-flowing powder wastransferred to tightly capped storage bottles.

Example 2

1102 g of non-stabilized lithium dispersion in oil (11.3%) thatcontained 124.5 g of lithium with a medium particle size of 45 micronwas charged under constant flow of dry argon at room temperature to a 2liter three neck glass flask reactor fitted with a stirring shaftconnected to a fixed high speed stirrer motor. 7.81 g of phosphoric acid(Aldrich) in a form of a 2 phase solution in 8 g of mineral oil at 21°C. was charged into the reactor. Temperature rise of 4° C. was notedalong with significant foaming and stirring continued for another hourand then transferred to the storage bottles. Further, lithium dispersionwas filtered and washed three times with hexane in an enclosed, sinteredglass filter funnel and twice with n-pentane to remove the hydrocarbonoil medium. The funnel was heated with a heat gun to remove traces ofthe solvents and the resulting free-flowing powder was transferred to atightly capped storage bottles.

Example 3

1128.5 g of non-stabilized lithium dispersion in oil (11.2%) thatcontained 126.4 g of lithium with a medium particle size of 63 micronwas charged under constant flow of dry argon at room temperature to a 5liter three neck glass flask reactor fitted with a stirring shaftconnected to a fixed high speed stirrer motor. 7.81 g of phosphoric acid(Aldrich) in a form of a 2 phase solution in 8 g of mineral oil at 20°C. was charged into the reactor drop-wise over the period of 6 minutes;higher agitation than in example 2 was used. Temperature rise of 4.5° C.was noted within 20 minutes, no foaming was observed, and stirringcontinued for another 5 hours and then transferred to the storagebottles. Further, lithium dispersion was filtered and washed three timeswith hexane in an enclosed, sintered glass filter funnel and twice withn-pentane to remove the hydrocarbon oil medium. The funnel was heatedwith a heat gun to remove traces of the solvents and the resultingfree-flowing powder was transferred to a tightly capped storage bottles.

Example 4

55.00 grams of non-stabilized lithium dispersion in oil (11.275%)containing 6.20 grams of lithium with a medium particle size of 63micron was charged into 120 ml hastelloy can equipped with a 1″ Tefloncoated stir bar. The solution was heated to 200° C. and 0.4 g AnhydrousH₃PO₄ previously melted in 2 ml of mineral oil was added to the lithiumdispersion. This mixture was continuously stirred at 200 rpm for 30minutes while holding the temperature at 200° C. Sample was allowed tocool to the room temperature and transferred to the storage bottle.Further, lithium dispersion was filtered and washed three times withhexane in an enclosed, sintered glass filter funnel and twice withn-pentane to remove the hydrocarbon oil medium. The funnel was heatedwith a heat gun to remove traces of the solvents and the resultingfree-flowing powder was transferred to a tightly capped storage bottles.

FIG. 1 demonstrates that process parameters affect the quality of thecoating. From left to right: amount of cavities/cracks reduced providingbetter hermeticity for the Li particle. Example 4 representsliquid/liquid reaction interface and is believed to provide even betterprotection: passivating layer is like a crust of microcrystallineLi₃PO₄. Adding, for example wax, will ensure that all the porosity,cracks, cavities are protected against moisture and atmospheric gases.

Example 5

52.3 grams of lithium dispersion in oil (12.0%), produced in example 1,containing 6.3 grams of lithium with a median particle size of 31 micronwas charged into 120 ml hastelloy can equipped with a 1″ Teflon coatedstir bar. 0.34 g of LuwaxS dry powder was also added to the can. Themixture was heated from ambient temperature to 75° C. at a rate of 5°C./min and held for 10 minutes. The sample was further heated from 75°C. to 175° C. at 5° C./min and held for one hour. Finally the mixturewas heated from 175° C. to 190° C. at a rate of 20° C./min followed byslow cooling to ambient temperature. This mixture was continuouslystirred at 200 rpm during the heating phase. Following cooling to theroom temperature the sample was transferred to a glass storage bottle.Further, the lithium dispersion was filtered and washed three times withhexane in an enclosed, sintered glass filter funnel and twice withn-pentane to remove the hydrocarbon oil medium. The funnel was heatedwith a heat gun to remove traces of the solvents and the resultingfree-flowing powder was transferred to a tightly capped storage bottles.

FIG. 2 illustrates a comparison of SEM images for Example 1 and Example5, and demonstrate the effect of a multi-coating approach.

Comparative Example 1

Battery grade lithium metal 441 grams was cut into 2×2 inch pieces andcharged under constant flow of dry argon at room temperature to a 3liter stainless steel flask reactor with a 4″ top fitted with a stirringshaft connected to a fixed high speed stirrer motor. The reactor wasequipped with top and bottom heating mantles. The reactor was thenassembled and 1215 g of Peneteck™ oil (Penreco, Division of the Penzoilproducts Company) were added. The reactor was then heated to about 200°C. and gentle stirring was maintained in the range of 250 rpm to 800 rpmto ensure all metal was molten. Then the mixture was stirred at highspeed (up to 10,000 rpm) for 2 minutes. Oleic acid, 4.41 g was chargedinto the reactor and high speed stirring continued for another 3minutes. Then the high speed stirring was stopped, heating mantlesremoved and dispersion was allowed to cool to about 100° C. at whichpoint 32.6 grams of fluorinating agent FC70 (perfluoropentylamine) wascharged into the reactor while stirring at about 800 rpm until cooled toabout 45° C. and transferred to the storage bottles. Further, lithiumdispersion was filtered and washed three times with hexane in anenclosed, sintered glass filter funnel and once with n-pentane to removethe hydrocarbon oil medium. The funnel was heated with a heat gun toremove traces of the solvents and the resulting free-flowing powder wastransferred to a tightly capped storage bottles.

Comparative Example 2

Battery grade lithium metal 441 grams was cut into 2×2 inch pieces andcharged under constant flow of dry argon at room temperature to a 3liter stainless steel flask reactor with a 4″ top fitted with a stirringshaft connected to a fixed high speed stirrer motor. The reactor wasequipped with top and bottom heating mantles. The reactor was thenassembled and 1215 g of Peneteck™ oil (Penreco, Division of the Penzoilproducts Company) were added. The reactor was then heated to about 200°C. and gentle stirring was maintained in the range of 250 rpm to 800 rpmto ensure all metal was molten. Then the mixture was stirred at highspeed (up to 10,000 rpm) for 2 minutes. Oleic acid, 4.41 g was chargedinto the reactor and high speed stirring continued for another 3minutes. Then the high speed stirring was stopped, heating mantlesremoved and dispersion was allowed to cool to about 100° C. at whichpoint 32.6 grams of fluorinating agent FC70 (perfluoropentylamine) wascharged into the reactor while stirring at about 800 rpm until cooled toabout 45° C. and transferred to the storage bottles. Further, lithiumdispersion was filtered and washed three times with hexane in anenclosed, sintered glass filter funnel and once with n-pentane to removethe hydrocarbon oil medium. The funnel was heated with a heat gun toremove traces of the solvents and the resulting free-flowing powder wastransferred to a tightly capped storage bottles. Physical properties forExamples 1-4 and Comparative Examples 1 and 2 are provided in Table 1.The table is shown that the physical properties demonstrate that theproperties are similar and there is no surface area effect.

Referring to FIG. 3, in the standard air stability test, Example 1clearly retained more metallic lithium. Lithium metal powder is spreadin a thin layer in the Petri dishes and exposed to certainmoisture/temperature conditions. Metallic lithium concentration ismonitored, the more metallic lithium is retained, the better thestability of the sample is.

TABLE 1 Comparison of Physical Properties Calculated SA, Coating TypeD50, micron m2/g Comparative Sample 1 Li₂CO₃ 32 0.22 Comparative Sample2 LIF 34 0.19 Example 1 Li₃PO₄ 31 0.20 Example 2 Li₃PO₄ 45 0.14 Example3 Li₃PO₄ 65 0.11 Example 4 Li₃PO₄ 63 0.11

Referring to FIG. 4, there is a comparison of the stability of Example 4and comparative example 1 in 0.6% water doped NMP is provided. This testshows that while CO₂-coated SLMP exhibits runaway reaction at about 48hours of exposure to the solvent doped with moisture, the SLMP producedaccording to the invention of example 4 has significantly improvedtolerance to moist NMP. The Example 4 SLMP does not have runawayreaction when exposed to room temperature for 72 hours and when exposedto 55° C. for about 30 hours.

Referring to FIG. 5, a comparison of the stability of samples producedaccording to Example 1 and Example 5 in NMP is provided. Test showsimmediate runaway was observed for the reaction system containingExample 1 sample while no runaway reaction was observed for the systemcontaining the Example 5 sample. The test was conducted at 30° C. for 24hours.

Having thus described certain embodiments of the present invention, itis to be understood that the invention defined by the appended claims isnot to be limited by particular details set forth in the abovedescription as many apparent variations thereof are possible withoutdeparting from the spirit or scope thereof as hereinafter claimed.

1. A method of providing a stable lithium metal powder comprising thesteps of: a) heating lithium metal powder to above its melting point toprovide molten lithium metal; b) dispersing the molten lithium metal;and c) contacting the dispersed molten lithium metal with aphosphorous-containing compound to provide a substantially continuousprotective layer of lithium phosphate on the lithium metal powder. 2.The method according to claim 1, wherein the step of heating the lithiummetal is conducted in a hydrocarbon oil.
 3. The method according toclaim 2, wherein the hydrocarbon oil is selected from the groupconsisting of mineral oil, petroleum oil, shale oils, and highly refinedoils.
 4. The method according to claim 1, wherein the step (c) ofcontacting the dispersed molten lithium metal with aphosphorous-containing compound is conducted during step (b) ofdispersing the molten lithium metal.
 5. The method according to claim 1,wherein the dispersed molten lithium metal of step (b) is cooled priorto step (c).
 6. The stable lithium metal powder produced according tothe method of claim
 1. 7. The stable lithium metal powder according toclaim 6, further including an organic coating.
 8. The stable lithiummetal powder according to claim 7, wherein the organic coating is a wax.9. The stable lithium metal powder according to claim 6, wherein thephosphorus-containing compound is selected from the group consisting ofphosphoric acid, P₂O₅ and POF₃.
 10. Stabilized lithium metal powderhaving a substantially continuous protective layer of lithium phosphate.11. The stabilized lithium metal powder of claim 10, further includingan organic coating layer.
 12. The stabilized lithium metal powder ofclaim 11, wherein the organic layer is a wax.
 13. A method of providinga stable lithium metal powder comprising the steps of: a) heatinglithium metal powder to above its melting point to provide moltenlithium metal; b) dispersing the molten lithium metal; and c) contactingthe dispersed molten lithium metal with a phosphoric acid to provide asubstantially continuous protective layer of lithium phosphate on thelithium metal powder.
 14. The method according to claim 13, wherein thestep of heating the lithium metal is conducted in a hydrocarbon oil. 15.The method according to claim 14, wherein the hydrocarbon oil isselected from the group consisting of mineral oil, petroleum oil, shaleoils, and highly refined oils.
 16. The method according to claim 13,wherein the step (c) of contacting the dispersed molten lithium metalwith a phosphoric acid is conducted during step (b) of dispersing themolten lithium metal.
 17. The method according to claim 13, wherein thedispersed molten lithium metal of step (b) is cooled prior to step (c).18. The stable lithium metal powder produced according to the method ofclaim
 13. 19. The stable lithium metal powder according to claim 18,further including an organic coating.
 20. The stable lithium metalpowder according to claim 19, wherein the organic coating is a wax.